1. Field of the Invention
This invention relates to a method of and apparatus for discriminating a variable transmission bit rate in a mobile communication system, and more particularly to a transmission bit rate discrimination method and apparatus for a variable bit rate mobile communication system (TIA.multidot.IS-95) wherein spectrum spreading which was standardized in the North America in July, 1993 is employed.
2. Description of the Related Art
Generally, in a mobile communication which employs spectrum spreading, when a base station communicates with a mobile station, it performs signal processing such as error correction and block interleaving (multiplexing) of a signal such as speech or data to be transmitted and then performs modulation of the signal using PSK (Phase Shift Keying) or the like, whereafter it spreads the spectrum in a wide bandwidth using a code such as a PN (Pseudo Noise) spread code (pseudonoise spread signal) and transmits the signal of the spread spectrum.
The data transmitted in this manner are despread, in the mobile station, using a PN code sequence same as and synchronized with that used in the base station and is then demodulated to obtain a signal of a base band, and the base band signal is processed by signal processing such as error correction and deinterleaving to extract an original signal of speech, data or the like.
In the TIA.multidot.IS-95, when a base station communicates with a mobile station, such signal processing as illustrated in FIG. 3 is performed in a traffic channel to transmit a signal.
Referring to FIG. 3, a variable bit rate is realized using four different transmission bit rates of 9.6 Kbps, 4.8 Kbps, 2.4 Kbps and 1.2 Kbps for data to be transmitted in a traffic channel. Information bits (101) which are transmission data to be transmitted at any of the transmission bit rates is first processed by a calculation of CRC (Cyclic Redundancy Check) data for error detection and a process (102) of adding a result of the calculation to the information bits (only when the transmission bit rate is 4.8 Kbps or 9.6 Kbps). Then, 8 bits are added as tail bits for convolutional encoding to the information bits (103).
The information bits at any of the transmission bit rates to which the CRC error detection data and the tail bits have been added are processed by convolutional encoding for error correction (104), and transmission symbols are transmitted repetitively in accordance with the transmission bit rate (105).
In this instance, the rate of repetitions of transmission symbols is zero at 9.6 Kbps, one at 4.8 Kbps, three at 2.4 Kbps and seven at 1.2 Kbps.
Thereafter, block interleaving processing (106) is performed, and using a long code generated by means of a 42-b PN code generator (1010), the transmission data are scrambled (107) by way of decimators (1011) and (1012). Further, information for power control is inserted into the transmission data (108) and the spectrum of the transmission data are spreaded in a wide bandwidth, and the transmission data are modulated (109). Then the modulated transmission data are transmitted.
In the mobile communication system which employs such spectrum spreading as described above, the transmission bit rate is discriminated by various methods in the mobile station. According to one method, the mobile station demodulates received data, despreads the demodulated data, descrambles the despread data, block interleaves the descrambled data to vary the order of the received data, performs Viterbi decoding corresponding to the four transmission bit rates, re-encodes results of the decoding using a convolutional encoder same as that of the base station, compares the re-encoded data with the data before the Viterbi decoding to detect correlations between them, and discriminates one of the results of the decoding which exhibits a maximum correlation as the data transmitted to the mobile station and discriminates the transmission bit rate from the discriminated transmission data.
However, if it is tried to discriminate a transmission bit rate using the conventional method described above, after Viterbi decoding is performed for the four different transmission bit rates, the decoded data must be re-encoded by convolutional encoders. Consequently, the conventional method is disadvantageous in that much time is required for the processing of the received data. Further, since convolutional encoders are required in each base station, the conventional method is disadvantageous also in that the circuit scale is increased as much, which makes an obstacle to miniaturization of the apparatus.
Various other techniques for coding/decoding transmission data are already known. One of known coding/decoding techniques is disclosed, for example, in Japanese Patent Publication Application No. Heisei 1-44056 wherein a high frequency self running clock is built in a Viterbi decoder so that internal signal processing can be processed serially in a time division relationship at a high rate. Another coding/decoding technique is disclosed in Japanese Patent Laid-Open Application No. Heisei 4-314289 wherein a decoding signal in a high definition television receiver is passed on to a quantization decoder which operates in response to a vector code book and a quantizing vector signal transmitted thereto and an output signal of the quantization decoder is passed on to an inverse discrete cosine transform (DCT) circuit. A further coding/decoding technique is disclosed in Japanese Patent Laid-Open Application No. Heisei 4-331519 wherein a format is conformed to different rates of audio encoders/decoders so that data can be transmitted at a variable rate in a same frame. However, those techniques cannot successfully eliminate the disadvantages of the conventional method described above.